350 mag mpi surging under load

silver_power

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Thank you very much. ...by the end of this week I will have further details concerning fuel pressure. Thank you very much.
 

silver_power

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Update: I measured myself the fuel pressure. Turn the key 34psi and as soon as the fuel pump stops 31psi. I turn the engine on and the pressure drops at 28psi. I got the boat on plane and the pressure steady at 28psi with the surging keeps on and the pressure steady to 28psi. While I push to accelerate the pressure rises to 30 and then to 28. I stop the boat with engine at idle and unhook the vacum hose of the regulator. The pressure rises to 34psi. I start the boat and accelerate with unhooked the vacum hose. The surging exists (steady 34psi again)but it is a little better and the engine has better response when accelerate. The pressure steady at 34psi. I returned at the dock and connect the vacum line to intake while the pressure drops at 28psi. I push the throtle to 1800rpm and I disconnect the vacum hose (again 34psi) The engine raised rpm to 2150rpm . I do not know finally what my mechanic measured. ???!!!! So I am sure that the regulator he changed he put 30psi instead of 43. With constant 34psi would the boat run normally or is normal that it surged again?
 

alldodge

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The surging is probably from the computer getting a knock or lean condition from a sensor and retarding timing or something else the ECM is saying this isn't right. so its compensating. In any case need to get the pressure back up
 

tpenfield

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Have the computer diagnostics ever been recorded while this engine is doing the surging thing?

​Walking around in the dark is getting nowhere.
 

silver_power

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Yes , one time I was driving the boat and mechanic had hooked the scan tool. It had not knock or retard while surging. According to ecu everything was fine. Anyway there is no reason for discuss if firstly I do not solve the fuel pressure issue and have clear 43psi at rail. I will change fpr and back to sea again for test. I will post as soon as I have news. Thank you very much.
 

silver_power

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I just put off the fuel pressure regulator that supposed mechanic changed because mine was rusty. It is the one with the pink mark part no:807952A1 and I think that it is the 30psi as I was imagine.....
 

silver_power

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I finally changed the fuel pressure regulator. The fuel pressure while pump work with no engine on is 43psi and when stops it is 41-42. When engine idles the pressure is 40psi and 46 with the vacum disconnected. The specs according manual is 43psi at 1800rpm. Is my results ok? I ask just because I expected the pressure when pump work and engine not, to be about 44-45 and when pump stop to be at 43....
 

alldodge

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I don't see any issue with your current results. The few psi variation can be from the accuracy of the gauge, and when shut off, can be from a slight leak. For right now I would give it a run
 

silver_power

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I will go for sure for a test in the sea. The pressure dropped only 1psi in about 5 minutes so I suppose that I do not have any leaks. I hope that the surging will stop.
I will post as soon as I test it in water.
 

ghunttwo

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I have a 350 mag mpi, 2003 and have been watching your problem . I had the same problem and was hoping that you solved yours . I bit the bullet and bought the Rinda Diacom program for my lap top. it showed no codes under load but over 2200rpms it would stumble and then shut off. it would restart with no problem, no codes.
​I had changed the shift interrupt switch and the gear position switch, no change. I plugged in the old shift interrupt switch and let it just hang there. the test run went perfectly with no stumbling and not shutting off. I plugged in the installed shift interrupt switch and it stumbled and shut off, I re tested several times and got the same results .
​I had changed the shift cables (both) and throttle cable some months back. what I have found is that the shift cables must be out of adjustment and ty wraped too tightly, just enough to give me a good shift but was causing the shift interrupt switch to activate just enough to cause the stumbling and then the computer would shut it down. Still no codes.
​I haven't had time to readjust the cables but I'm confident that I found the problem.

​Try jumping the shift interrupt switch with a jumper or a Bravo shift switch jumper.
Good luck, hope this helps

George
 

tpenfield

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I would think on the Diacom software with an interrupt activating intermittently, you would see all of the data go flat line . . . of course the Diacom may loose connectivity at that point.

​Seeing codes, is only a small part of what the Diacom software is good for. Seeing the data in real-time and being able to record it and play it back on graphs, etc. is the more significant value.
 

ghunttwo

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Yes , when the engine quits all values go to ing on, eng off. Even running the Diacom recording grafph's at the slowest speed there is no indication or codes that show interrupter activation. All graphsdrop to "0" all at the same millisecond.
I just thought the OP might try bypassing the interrupt switch to see if that corrects his problem.
Bypassing that switch is cheeper than replacing a bunch of expensive parts.
Thsts what I've found with mine and now that I know what the problem is, I can fine tune the shift cables adjustment to fix it.
 

QBhoy

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I might hazard a guess at the throttle position sensor....well known MPI problem area...amongst others.
 

ghunttwo

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It was fooling me as well , thats why I bought the Diacom , and the 555 manual. It takes a lot of reading and studying the pages of codes and guardian action to those codes. In my.case the only guardian eng shut down is code193, shift antisapation switch fault
all the others are reduction of power less than 100%.
And that code doesn't show up on Diacom.
Hope this sheds a little light on our friends problem.

George
 

silver_power

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I have not post yet any news just because the weather where in Greece is not good and so I have not test the boat with the correct fuel pressure regulator I recently changed. My mech installed a 30psi instead of 43psi. Now, concerning the shift interrupt problem , I don't think this is my problem because the boat never stoped while it is running. Anyway, I hope this week I have further news after test in to the sea.
 

ghunttwo

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I hope after all of your time spent looking for your problem you have a successful sea trial.
 

QBhoy

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Would be really keen to hear how you get on.
I?ve had similar problems and symptoms.
The surging, 90% power restrictions and spurious sensor errors like TBC. Knock sensor, IAC, SW pressure and miss fire errors on all cylinders.
I?ve reolaced the diz cap and rotor, then IAC etc and a full reset of ecm by dealer. Runs well most of the time, but occasionally flickers and surges near wot and won?t give the full beans 100% of the time. When it does it?s a machine...just won?t reliably give full potential all of the time. Haven?t really solved it. Haven?t replaced the TBC or SW pressure switch, but they will be next on the list.
 

Fun Times

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I have a 350 mag mpi, 2003 and have been watching your problem . I had the same problem and was hoping that you solved yours . I bit the bullet and bought the Rinda Diacom program for my lap top. it showed no codes under load but over 2200rpms it would stumble and then shut off. it would restart with no problem, no codes.
​I had changed the shift interrupt switch and the gear position switch, no change. I plugged in the old shift interrupt switch and let it just hang there. the test run went perfectly with no stumbling and not shutting off. I plugged in the installed shift interrupt switch and it stumbled and shut off, I re tested several times and got the same results .
​I had changed the shift cables (both) and throttle cable some months back. what I have found is that the shift cables must be out of adjustment and ty wraped too tightly, just enough to give me a good shift but was causing the shift interrupt switch to activate just enough to cause the stumbling and then the computer would shut it down. Still no codes.
​I haven't had time to readjust the cables but I'm confident that I found the problem.

​Try jumping the shift interrupt switch with a jumper or a Bravo shift switch jumper.
Good luck, hope this helps

George
Without trying to highjack silver_power's topic and we all await his latest testing's......I'll just mention in passing only that being you have an 2003 engine, your issues became known to Mercruiser and the repair for them was to replace the ECM....That mentioned, when I went looking for similar type issues you're having and while talking with Merc after the warranty timeframe expired, one of the service tech supports mentioned (described) what was happening was the ECM sensitivity was programmed to sensitively for reading (or sensing) the shift interrupt switch "vibration" while it was mounted on/to the engine mounted shift cable plate assembly.

One of his suggestions was to if possible, remove the shift plate assembly from the engine and try mounting it to the boats rear transom wall area as they seemed to have noticed removing the shift plate assembly from the engine to the transom area helped with this particular issue.

Pretty good you thought to try another switch just plugged in but not mounted and it seemed to help solve the problem....You must be very tech or engineered minded.:)

Your cable theory of being to tight somewhere along the lines seems very probable as it could be maybe vibrating the cable lines just enough activating the shift switch somehow.

While the ECM is connected to the shift interrupt switch and they talk to each other, I believe you'll notice that you won't be able to read on your marine scan tool hardware if the "shift" interrupt switch is being activated/deactivated causing issues with your engine running......The only switch you'll be able to read on the scan tool would be the "gear" indicator switch which reads if you're in or out of gear.... So in the following link, the scan tool will show/read item number 20 but not number 18, http://www.mercruiserparts.com/bam/subassembly/31749/11652/300

Also bear in mind that there is a specific measurement that's supposed to ride between the switch plunger and the V of the shift plate Actuator lever.

If you don't find your tight cable theory to be the case then another test could be maybe you could try coming up with someway of adding something to help absorb possible vibrations to the whole shift plate assembly...Of course first unbolt the plate assembly from the engine and see if it helps any and go from there.

Page 3 of the following bulletin was basically the test you tried with a new switch not mounted, Service Bulletin - Marine Mechanic

Hope this helps some, good luck.
 

cableguy1979

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Jun 25, 2017
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I had a boat that did this and it about drove me nuts...it wind up being something got caught up in the anti siphoning valve on the gas tank....it had to be replaced because it had a ball bearing that rusted amd stuck. It allows only a certain amount of fuel in.
 
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